1. Field of the Invention
The present invention relates to a gas ball valve, and more particularly to a gas ball valve which maintains a seal under high temperatures.
2. Background of the Art
There are two general types of gas service line valves: a metal-to-metal seated valve and a soft seated valve. The metal-to-metal seated valve provides a metal against metal seal by means of lubricant and/or sealant. Unfortunately, the metal-to-metal seated valve requires periodic maintenance to ensure a tight seal and low turning torque. In contrast, the soft seated valve provides a resilient material against metal seal. Although the soft seated valve is relatively maintenance free and provides a lower turning torque, it has a disadvantage in that it fails to function properly in a high temperature environment because the resilient material does not maintain its sealing integrity in high temperatures.
There is an increasing demand for a gas line ball valve which has the advantages of a soft seated arrangement (i.e., low maintenance and easy turning) and is capable of functioning properly in a high temperature environment (such as a fire) as provided for by the requirements of the American Petroleum Institute (API) No. 607. In a high temperature environment, it is important that the gas ball valve be easily turned off to prevent further flow of gas into that environment. Moreover, once the valve is off, it is critical that the valve maintain a minimum seal as defined by API 607 which presently is the only national standard used to evaluate the fire resistance of a valve. Gas industry expectations also may include sealability at pressures less than those used in API 607. Such a valve would have a number of applications including use for gas distribution to residential and business establishments, and use in petrochemical and refinery plants.
Legal regulations prevent the use of soft seated valves it the valves ability to control a flow of gas is adversely effected by exposure to anticipated heat, which may include a fire (see, for example, 49 CFR 192.363). Recent interpretations by OPS have relied upon API 607 as the standard to be used to demonstrate resistance to anticipated heat. With the API 607 standard, one must employ tests for leakage at a pressure that is 75% of the valve pressure rating.
Previous soft seated valves were incapable of operating in a high temperature environment because the resilient material (such as rubber) tended to melt or deform, thereby permitting gas to leak through the seal. Therefore, valves employed in high temperature environments exclusively used seals made of heat resistant materials, such as polytetrafluoroethylene resins, a material that is typically harder than common rubber seals. Such high temperature seals are relatively expensive. Although these high temperature seals performed adequately under temperatures up to 600.degree. F., the seals had a disadvantage in that they did not always perform well at low gas pressures ( at some pressure less than the 75% of the valve pressure rating API 607). In other words, the high temperature seals performed satisfactorily during the high temperature tests because the tests are performed at line pressures of 75% of rated pressure; but, the high temperatures seals were sometimes found to leak at low line pressures. In practice, line pressures may be very low, particularly when the valve is used on a service line to a residential or business establishment. Therefore, the valves employing high temperature seals are not well suited to perform in all environments and at all line pressures. Specifically, a valve in actual practice may be subjected to a lower temperature than that set forth in API 607 and/or to a lesser pressure than 75% of the valve rated pressure. If the high temperature seal does not fully melt when heated and/or if the pressure is not sufficient to fully deform the high temperature seal so as to move the seal out of the closing path of the valve, a valve may fail to meet the API 607 standard.
Another disadvantage of conventional gas valves is the expense of manufacturing. One expense is the added cost of using high temperature seals. Another expense is that conventional gas valves have caps which are permanently fixed to the valve stem. These caps are used to operate the valve and may be secured to prevent someone from turning the gas on or off without authorization. Unfortunately, the cost to manufacture these caps is a considerable percentage of the cost to manufacture the entire valve. Therefore, providing a cap for each valve significantly increases the per unit cost of each valve.
Another problem encountered by conventional gas valves is vandalism or theft. Conventional fixed caps are usually made from a soft brass or other non-hardened material which is ideal for the operating mechanism of the valve. The softer materials, however, can be easily cut with a hacksaw or the like, thereby permitting vandals or thieves to tamper with the gas valve.